Yes your treatment for cancer is your own choice. We are all free to choose a high risk of dying soon against the best advice, they will not force us to take their anti-cancer treatments. You are free to delay Tamoxifen against your oncologists recommendation, you are free not to take it at all.
But please first ask your oncologist what difference it will make to your risk of the cancer coming back.You may basically be choosing a very high choice of an early death if you make this choice. And for our friend, it is one she deeply regrets now she has little time left to live because of that choice.. She is so upset that she let those distressingmenopausal symptoms etc matter more then than continued life now
If your cancer is/was Estrogen positive, Tamoxifen modifies your body so it is no longer a nourishing environment for the "seed" cancer cells you almost certainly have in your blood post surgery and for years later,. These have the potential to grow into recurrent or metastatic cancer. Metastatic breast cancer is fatal for over 80% of patients who get it, lower for HER2+cancers that have not previously received herceptin, but still too high for comfort.
Please do not kid yourself that your surgery and radiotherapy and the fact you have no evidence of disease means that you are totally free from cancer cells that may yet grow into new cancers, as happened to our friend.
The higher the % Estrogen-positive your tumor cells are, the more likely the cancer stem cells are to regrow into a new metastatic cancer over the next 10 years. My cancer is under 2% Estrogen positive, so Estrogen is not a fuel for my cancer. Instead mine is strongly HER2+, -ER-,PgR-,so herceptin blocks off the HER2recieptors so the growth hormone cannot get in to my cancer cells and boost their growth. If like me you have a very low ER+ percetage, there is nothing to be gained by taking this drug and risking the side effects. But if it is higher (and yr surgeon/oncologist will tell you what figure means Tamoxifen will help you) it could be the difference between long life and an early death for you,
HER2+ cancers grow very fast, and the seed cells almost always"germinate" into recurrent/metastatic cancers within 5yrs if they are going to.
But studies of who gets the cancer back when is showing that ER+ seed cells are viable for much longer than the just HER2+ ones, so if you stop taking Tamoxifen after 5yrs, you still have too high a chance there will be some that will sprout and grow at that point
I hope you make well-informed choices that work for you and give you both a long healthy life and a life free from distressing symptoms. I also hope that if you have to choose between these two, you will make sure you really know the risks you are taking, and that you won't later regret the choice you make now.